1. Field of the Invention
The present invention relates to microwave ovens and, more particularly, to a microwave oven designed to improve a sensing performance of its humidity sensor used to sense humidity in a cooking cavity of the microwave oven by sensing the humidity of air exhausted from the cooking cavity, and to a method of controlling the microwave oven.
2. Description of the Related Art
A microwave oven is an electrically operated oven using a magnetron to generate high-frequency electromagnetic waves. The high-frequency electromagnetic waves have a fundamental frequency of 2450 MHz, and are radiated into a cooking cavity to repeatedly change the molecular arrangement of moisture laden in food and generate intermolecular frictional heat within the food to cook the food.
In recent years, in order to meet a variety of requirements of consumers, a microwave oven with a humidity sensor has been proposed and used. In an operation of such a microwave oven, the humidity sensor senses the humidity of air inside the cooking cavity, and automatically controls the cooking process according to the sensed humidity.
FIG. 1 shows a conventional microwave oven with a humidity sensor 6. A body 1 of the microwave oven is partitioned into a cooking cavity 2 and a machine room 3. A door 4 is hinged to the body 1 so as to close the cooking cavity 2. The microwave oven also has a control panel 5, which is installed at a front wall of the body 1 and is provided with a variety of control buttons. The humidity sensor 6 is installed in the body 1 to sense the operational conditions of the food being cooked in the cooking cavity 2.
The cooking cavity 2 is opened at its front and has a turntable-type cooking tray 2a rotatably mounted on a bottom of the cooking cavity 2. An air inlet 7a is formed at a front portion of a sidewall 7 of the cooking cavity 2 so as to allow the cooking cavity 2 to communicate with the machine room 3. Air flows from the machine room 3 into the cooking cavity 2 through the air inlet 7a. An air outlet 8a is formed at a rear portion of an opposite sidewall 8 of the cooking cavity 2 so as to discharge air from the cooking cavity 2 to the atmosphere exterior to the body 1.
A magnetron 3a, a cooling fan 3b, and an air guide duct 3c are installed within the machine room 3. The magnetron 3a generates the high-frequency electromagnetic waves, while the cooling fan 3b sucks atmospheric air into the machine room 3 so as to cool the elements such as the magnetron 3a installed within the machine room 3. The air guide duct 3c guides air inside the machine room 3 to the air inlet 7a. The cooling fan 3b is installed at a position between the magnetron 3a and a rear wall of the machine room 3. In order to allow atmospheric air to flow into the machine room 3 from outside the body 1, a predetermined area of the rear wall of the machine room 3 is perforated to form a plurality of air suction holes 3d. 
The humidity sensor 6 is installed on the sidewall 8 of the cooking cavity 2 at a position adjacent to the air outlet 8a such that it is placed in an air discharging passage leading from the cooking cavity 2. The humidity sensor 6 senses the humidity of exhaust air discharged from the cooking cavity 2 through the air outlet 8a. The humidity sensor 6 is connected to a circuit board (not shown) installed in the control panel 5, and outputs a signal to the circuit board.
When turning on the microwave oven containing food on the cooking tray 2a by manipulating the control panel 5, the high-frequency electromagnetic waves are radiated from the magnetron 3a into the cooking cavity 2 to cook the food. During such an operation, the cooling fan 3b is rotated to form a suction force which draws the atmospheric air into the machine room 3 through the air suction holes 3d and cools the elements such as the magnetron 3a installed in the machine room 3. The air is, thereafter, guided to the air inlet 7a by the air guide duct 3c and introduced into the cooking cavity 2 through the air inlet 7a. The air inside the cooking cavity 2 is exhausted along with vapor generated from the food being cooked to the atmosphere through the air outlet 8a as shown by the arrows of FIG. 1. Therefore, it is possible to remove odor and vapor generated from food during the operation of the microwave oven.
When the exhaust air flows from the cooking cavity 2 to the atmosphere, it comes into contact with the humidity sensor 6. The humidity sensor 6 senses the humidity of the exhaust air, and outputs a signal to the circuit board of the control panel 5. To automatically cook the food on the tray 2a, the circuit board of the control panel 5 controls the operation of the magnetron 3a, the cooking tray 2a and the cooling fan 3b in response to the signal from the humidity sensor 6.
However, the conventional microwave oven is problematic due to the humidity sensor 6 being installed at a position close to the air outlet 8a which discharges air from the cooking cavity 2 to the atmosphere. Specifically, when the microwave oven sequentially performs several cooking processes, the air inside the cooking cavity 2 is excessively heated and discharged to the atmosphere through the air outlet 8a. Thus overheating of the humidity sensor 6, which reduces the sensing performance of the humidity sensor 6. In addition, moisture and contaminants, such as oil and smoke, generated from the food during the cooking processes are deposited onto the surface of the humidity sensor 6 as the moisture and contaminants flows from the cooking cavity 2 to the atmosphere along with the exhaust air through the air outlet 8a. The moisture and contaminants deposited on the surface of the humidity sensor 6 are not easily removed from the humidity sensor 6, and the sensing performance of the humidity sensor 6 is reduced.
Furthermore, when the amount of food in the cooking cavity 2 is large, the air does not smoothly circulate in the cooking cavity 2. Thus, the amount of exhaust air discharged from the cooking cavity 2 through the air outlet 8a is reduced. In such a case, the sensing performance of the humidity sensor 6 installed outside the air outlet 8a is remarkably reduced. Therefore, a microprocessor (not shown) set on the circuit board of the control panel 5 is unable to precisely determine the cooked state of the food, and the food is either undercooked or overcooked.
Accordingly, it is an object of the present invention is to provide a microwave oven with an improved air outlet and humidity sensor mounting structure to prevent the humidity sensor from being overheated or contaminated by air exhausted from a cooking cavity, and to allow the humidity sensor to precisely sense the humidity of the air inside the cooking cavity during a cooking process.
Another object of the present invention is to provide a microwave oven which controls the air flow speed inside a cooking cavity to improve the sensing performance of a humidity sensor, and a method of controlling the operation of the microwave oven.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided a microwave oven according to an embodiment if the present invention includes a control unit which determines the conditions of food being cooked in response to automatically or manually inputted information and controls the rotational speed of a cooling fan in response to the determined conditions of the food to improve the sensing performance of a humidity sensor.
According to another embodiment of the present invention, a method of controlling a microwave oven includes receiving input information of food to be cooked, determining the rotational speed of a cooling fan in accordance with the conditions of the food determined by using the input information, and operating the cooling fan at the determined rotational speed and cooking the food.